Introduction
Grassland is a central component of the terrestrial ecosystem, playing a
prominent role in maintaining the world’s ecological balance (Gravuer et
al., 2019; Prommer et al., 2020). Grasslands across arid and semi-arid
regions are currently undergoing widespread shrub encroachment, which
were described as the rise in dominance and abundance of shrubby plants
in grasslands (Stevens et al., 2017; Aguirre et al., 2021). These
ecosystem transformations may be caused by combinations of climatic
variation and anthropogenic processes, including large increases in
precipitation, air temperature, nitrogen deposition, as well as grazing
disturbances (Formica et al., 2014; Pistón et al. 2016). Shrub
encroaching into grasslands often caused abrupt variations in
biodiversity, vegetation productivity, hydrological properties, carbon
dynamics, and soil nutrients (Liu et al., 2021; Broadbent et al., 2022).
These changes are closely linked with alterations in not only the
overall but also above- and belowground ecosystem functioning (Eldridge
et al., 2011; Valencia et al., 2015).
Alpine ecosystem on the Tibetan Plateau has undergone rapid climate
changes and intensity anthropogenic activities in recent decades (Kuang
et al. 2016), which are also associated with substantial structural and
functional variations of alpine ecosystems, particularly the prevalently
ecological process known as shrub encroachment (Wu et al., 2021; Zhao et
al., 2023). In general, compared with herbaceous species, shrub species
with greater aboveground and root biomass can provide more organic
matter input soils (Zhou et al. 2017; Cui et al., 2023) and enhance the
accumulations of soil carbon and nitrogen (Li et al., 2019; Zhao et al.,
2023). Shrub encroached into alpine grassland ecosystems can increase
biodiversity, soil fertility, soil infiltration processes, ultimately
attributing to strengthen ecosystem functioning (Butterfield et al.,
2016; Cai et al., 2020). Nevertheless, neutral or even negative
ecosystem individual and multiple functioning response to shrub
encroachment also existed in alpine ecosystems (Zhang et al., 2022; Yang
et al., 2023). Thus, the ecological impacts of shrub encroachment are
expected to affect biodiversity and even lead to the shifts of
ecological functioning in alpine ecosystems, which depend on a variety
of factors, including shrub traits, edaphic condition, and local climate
(Brandt et al., 2013; Collins et al., 2020).
In semi-arid alpine grasslands, increasing evidence suggesting that
shrub encroachment can have the potential positive effects on the
ecosystem functions, including plant diversity, soil hydrological, and
soil fertility (Noumi et al., 2016; Wang et al., 2023). Nevertheless,
the degree by which shrub encroachment caused variations in the
ecosystem functioning could vary with the traits of encroaching shrubs
(e.g., Leguminosae or Non-Leguminosae species). In semi-arid regions,
Leguminosae shrubs can overcome drought stress and nutrients constraints
to promote species richness, vegetation productivity, and carbon
accumulation (Saixiyala et al., 2017; Ale et al., 2023). However, the
encroachment of Leguminosae shrubs was differently associated with the
ecosystem functioning than that of none-leguminous shrubs (Li et al.,
2016; Zhao et al., 2023). Nevertheless, their wider positive impacts on
belowground ecosystem functioning, and the underlying mechanisms and the
changes magnitude of the ecosystem functioning of the semi-arid alpine
steppes induced by shrub encroaching, remain poorly understood.
Much less attention has been given to recent widespread encroachments of
shrubs in semi-arid alpine ecosystems, such as leguminous shrub
(Caragana spinifera ) and none-leguminous shrub (Dasiphora
fruticosa ). These encroaching shrubs was expected to provide favorable
habitat for herbaceous plants due to increased shrubs abundance and
sprawling canopy that provide increased ultraviolet radiation protection
and buffering of extreme soil temperatures and drought stress, as well
as an increase in soil nutrients availability (Cui et al., 2023; Zhao et
al., 2023), which are associate with strengthening ecosystem
functioning. These dwarf shrubs may be associated with different above-
and belowground effects, including increased vegetation productivity and
increased soil nutrient contents. We tested two hypotheses. First, we
hypothesized that encroachment of both Leguminosae and non-Leguminosae
shrubs had beneficial effects on the ecosystem functioning of the
semi-arid alpine steppes, which was associated with increased vegetation
productivity and higher soil nutrient availability. Second, we
hypothesized that above- and belowground ecosystem functioning of the
semi-arid alpine steppes responded differently to shrub encroachment,
with the belowground ecosystem functions increased greatly than the
aboveground ecosystem functions.
Here two shrub encroached alpine steppe sites of the central Tibetan
Plateau were surveyed, with one site encroached by a leguminous shrub
(Caragana spinifera ) and the other site encroached by an
none-leguminous shrub (Dasiphora fruticosa ) . The changes in the
individual and multiple ecological functions of the semi-arid alpine
steppes following the encroachments of leguminous shrub and
none-leguminous shrub were assessed. The aims of our study were to (1)
quantify the impacts of encroached by leguminous shrub and
none-leguminous shrubs on above- and belowground ecosystem functioning
of semi-arid alpine steppes, and (2) identify the underlying mechanisms
of encroaching shrubs induced variations in ecosystem functioning in the
semi-arid alpine steppes.